Can someone verify that this math is correct? If your traveling up or down on an isometric map the speed of the character should be half that of traveling horizontal. What I am not sure about is traveling at an angle. The character speed should be the horizontal and vertical speeds divided together correct? So 0.75. This will be multiplied times the actual character speed.

Hmmm... Going to go see if I can find some information on eight directional isometric character movement speeds...

Attached Thumbnails

Here is something you should copy from the 3d world: world space and view space.
You should do all your game logic in world space, which should not differ between moving up or left. The view space is what you currently see and you need two functions to convert between the two spaces.
world2View(x,y) := (x-y*0.5,x+y*0.5) + (screenoffset_x,screenoffset_y)
view2World(x,y) := ...homework...

The world2View transformation is only needed for rendering your scene, please remember, that your world is somewhat turned by 45 degree, therefore you just can't make 1:1 conversion. The other way around (view2World) is needed whenever you want to convert input (e.g. mouse input) back to your world space.

There's some math involved which isn't hard, but makes the handling much easier. Once you work in world space independently of the screen lot of issues just vanish, like your movement example

According to my calculations angular speed should be... 0.55978260% of Horizontal. So a multiplier of 0.56 would be correct? Hmmm... That doesn't sound right... I think I need to calculate out non isometric tile distances first to solve this easier...